Geographical and seasonal variations in taxonomic composition, abundance and biomass of microzooplankton across a brackish-water lagoonal system of Japan

The taxonomic composition, abundance and biomass of microzooplanktonwere studied at eight stations in Lake Shinji–Ohashi River–LakeNakaumi brackish-water system, Japan, monthly from April 1998to March 1999. Over the entire area, naked ciliates numericallydominated the microzooplankton community (annual mean 39.6%)followed, in order, by tintinnids (30.3%), copepod nauplii (24.6%)and rotifers (5.5%). The abundance of each taxonomic group ofmicrozooplankton varied geographically due to large salinityvariations (range 1.5–33.3 p.s.u.). It was notable thatnaked ciliates occurred overwhelmingly in Lake Shinji (54.9%of total microzooplankton) and rotifers were relatively numerousin Lake Shinji (8.8%) and Ohashi River (11.1%), where the salinitywas lower (annual mean 4.1 and 13.6 p.s.u., respectively) thanin Lake Nakaumi and Sakai Strait (26.3 and 29.8 p.s.u., respectively).Owing to large seasonal temperature variation (range 5.4–29.8°C),the abundance of microzooplankton showed marked seasonal variations,being higher in spring and summer than in the remaining seasons.A total of 49 species of tintinnids were identified, and 15of these species reached concentrations >500 individualsl^-l. The occurrence of most tintinnid species was confined tocertain months or locations, closely associated with species-specifictemperature and salinity preference and/or tolerance. In thiseutrophic system, food supply for microzooplankton might besufficient due to extremely high chlorophyll a concentration(annual mean 8.8 µg l^-lin Sakai Strait to 22.6 µgl^-l in Lake Shinji). However, microzooplankton biomass remainedmoderate (range 0.19–18.7 µg C l^-l) due probablyto heavy predation by mesozooplankton, which inhabit this brackish-watersystem at extremely high biomass.